1. Field of the Invention
This invention relates to a fluidized bed reactor for preparing a metal nitride of high purity on an industrial scale.
2. Prior Art
Fluidized bed reaction is often utilized for the preparation of nitrides of metals such as silicon, aluminum and boron. A typical reactor used for such reaction is shown in FIG. 2. The reactor includes a reaction tube 1 in which a fluidized bed 2 is formed. To the lower end of the reaction tube 1 is connected a gas inlet tube 5 having a gas inlet port 4 through a diffuser plate 3. To the upper end of the reaction tube 1 is attached a lid plate 7 having a gas outlet port 6 through an O-ring 8. Outside the reaction tube 1 is disposed a heater 9 which surrounds the reaction tube 1. The heater 9 is, in turn, surrounded by a heat insulator 10. The reaction tube 1 is charged with metal powder such as Si, Al, and B powder. A non-oxidizing gas containing N.sub.2 or NH.sub.3 is introduced into the reaction tube 1 from the gas inlet port 4 through the gas inlet tube 5 and apertures in the diffuser plate 3, to thereby form a fluidized bed 2 with the metal powder. The heater 9 is actuated to heat the fluidized bed 2 to a temperature of at least 1,200.degree. C. for effecting nitriding of the metal powder.
In reactors as shown in FIG. 2 for preparing metal nitrides through fluidized bed reaction, the nitriding temperature is as high as 1,200.degree. C. or more. To avoid reaction between the reaction tube 1 and the metal powder, a metallic tube is not generally used. Commonly used reaction tubes are of ceramic materials such as SiC, Si.sub.3 N.sub.4, mullite and Al.sub.2 O.sub.3. Ceramic tubes, however, can be poorly worked or machined as is well known in the art, and it is difficult to accomplish tight seals. The top and bottom ends of a reaction tube are sealed using an O-ring as shown in FIG. 2 or a glass cap. The resultant seals are still unsatisfactory.
As a result of deficient seals, the prior art reactors have the following problems.
(1) The surrounding air diffuses and penetrates into the reaction tube to form an oxide film on a surface of metal particles which becomes a barrier against diffusion of N.sub.2 gas, inhibiting effective reaction. Since oxygen will form a solid solution during nitriding, the resulting metal nitride is of low purity. A sintered product of such metal nitride will have low strength. PA1 (2) One common approach for removing oxide film on the surface of starting metal powder is to mix a reaction gas such as N.sub.2 or NH.sub.3 with hydrogen gas and feed the mixture into the fluidized bed. There is a possibility that explosive hydrogen gas leak out of the reaction tube. PA1 (3) Reaction tubes of ceramics are less resistant to thermal shock. Upon failure of the reaction tubes by thermal shock, the fluidized bed contents at high temperature will flow out of the system. PA1 (4) As fines of metal powder ground in the fluidized bed will scatter out of the system and the vapor of metal powder will diffuse, they deposit on or react with the surface of the heater and thermal insulator, substantially reducing the life thereof.
One solution is proposed in Japanese Patent Application Kokai (JP-A) No. 97110/1986 as a reactor arrangement wherein a reaction tube and a heater are enclosed with an outer enclosure for shutting off the surrounding atmosphere. This reactor arrangement, however, is difficult to continuously operate on an industrial scale. Since the upper end of the reaction tube is open, metal powder fines formed in the fluidized bed will scatter out of the reaction tube and the vapor of metal powder will diffuse. They deposit on the surface of the heater and thermal insulator to adversely affect them, reducing the life thereof.